1. Field of the Invention
This invention relates to a biodegradable plastic composition and a biodegradable plastic shaped body formed of such a composition.
2. Description of Prior Art
To cope with increasing problems of environmental pollution by plastic wastes, various studies have been and are still intensively made on plastic shaped bodies which are degradable or disintegrable upon being placed in an environment where microorganisms can grow. From the stand point of preventive pollution, it is highly desirable that a plastic shaped body be completely biodegradable.
Polycaprolactone (hereinafter referred to as PCL for brevity) is known to be biodegradable. However, PCL is low in modulus of elasticity and is poor in thermal resistance, though the impact strength thereof is excellent. Poly-.beta.-hydroxy-butyrate (hereinafter referred to as PHB for brevity) is also known to be biodegradable. However, PHB is difficult to mold and low in impact strength, though the tensile and bending strengths and the modulus of elasticity thereof are high. Thus, PCL and PHB have not yet been widely utilized for biodegradable shaped bodies.
Japanese Published Unexamined Patent Application No. H3-157450 (equivalent to U.S. Pat. No. 5,124,371) discloses a biodegradable plastic composition including a blend of PCL and PHB. While this prior art composition exhibits good biodegradability, it has been found that the mechanical properties thereof are not fully satisfactory. In particular, it has been found that when PCL and PHB are used in the same amount according to the working example of this prior art, variation in mechanical properties and appearance is seen between respective shaped bodies as well as in each of the shaped bodies. This is considered to be attributed to difficulty in obtaining a homogeneous mixture of PHB and PCL. To overcome the difficulty, this prior art suggests the use of a PHB-PCL copolymer which serves to improve compatibility between PHB and PCL.
While the use of such a copolymer can improve the compatibility, another problem is caused because it is difficult to control the content of the copolymer and the properties of the shaped product obtained from the composition and because the copolymerization catalyst unavoidably remains in the composition. Further, the presence of the copolymer causes a reduction of the crystallization rate of a melt of the composition so that, in the molding of the melt, a relatively long cooling time is required for completing the crystallization. Moreover, the copolymer adversely affects the gas barrier property of the molded product because the crystallinity thereof is lowered.